LECTURE-04
DECISION MAKING &
BRANCHING
Decision-making:
• Decision-making structures require that the programmer specifies one or more conditions to be evaluated or tested by the program, along with a statement or statements to be executed if the condition is determined to be true, and optionally, other statements to be executed if the condition is determined to be false.
• Shown below is the general form of a typical decision- making structure found in most of the programming languages: C
C programming language assumes any non-zero and non-null values as true,
and if it is either zero or null, then it is assumed as false value.
C language possesses decision making capabilities by supporting the following statements:
i. if statement
ii. switch statement
iii. conditional operator
• Since these statements control the flow of execution, they are also known as control statements.
i. if statement
It takes the following form:
Entry
Test Expression
?
False
True
if (test expression)
The different forms of if statements are:
1. Simple if statements 2. if……..else statements
3. Nested if…….else statements 4. else…….if ladder.
A selection structure is used to choose among alternative courses of action.
if student’s grade is more than 40 print “passed”
if grade of a student is more than 40 then the print command will be executed. Otherwise, if the student’s grade is not more than 40, the compiler will move to next
So, as a general form, we can see the if selection structure as-
if (condition) { body of if }
The condition needs to be true to get into the body of if.
Otherwise, the compiler will go to the next segment of codes.
1.Simple if statements
It takes the general form.
If (test expression) {
Statement block;
}
Entry
Statement-block True
False
Statement-x Test
expression ?
2. if……..else statements
It takes the general form:
if ( test expression) {
true block statement;
} else {
false block statement;
}
statement-x;
Entry
False
Statement-block
True False
Statement-x Test
expression ? True
Statement-block
The if selection structure performs an indicated action only when the condition is true. Otherwise, the action is skipped. The if/else structure allows the programmer to specify that different actions are to be performed when the condition is true and when the condition is false.
if student’s grade is more than 40 print “passed”
else
print “failed”
If the condition of if is true, then compiler will print passed and if the condition of if is false, the compiler will print failed. So, as a general form, we can see the if/else selection structure as-
if (condition) {
body of if }
Else {
body of else }
Note- An if structure may not have any else
statement followed by it but an else structure
must have an if structure preceded.
3. Nested if…….else statements
An entire if-else construct can be written within either the body of the if statement or the body of an else statement. This is called
‘nesting’ of ifs.
This is shown in the following structure.
if (n > 0) {
if (a > b) z = a;
} else z = b;
The second if construct is nested in the first if statement. If the condition in the first if statement is true, then the condition in the second if statement is checked. If it is false, then the else statement is
Now, consider a big scenario where you may require a nested if else structure.
if student’s grade is more than 90 print “Grade: A”
else
if student’s grade is more than 80 print “Grade: B”
else
if student’s grade is more than 70 print “Grade: C”
else
if student’s grade is more than 60 print “Grade: D”
else
if student’s grade is more than 50
4. else…….if ladder.
This sequence of if statements is the most general way of writing a multi−way decision. The expressions are evaluated in order; if an expression is true, the statement associated with it is executed, and this terminates the whole chain. As always, the code for each statement is either a single statement, or a group of them in braces.
If (expression) statement
else if (expression) statement
else if (expression) statement
else if (expression) statement
else statement
The last else part handles the ``none of the above'' or default case where none of the other conditions is satisfied.
Sometimes there is no explicit action for the default; in that case the trailing can be omitted, or it may be used for error checking to catch an “impossible” condition.
It is always legal in C programming to nest
if-else statements, which means you can
use one if or else if statement inside
another if or else if statement(s).
• The Switch Statement:
In real life, we often have to make decisions from number of alternatives- which country to visit, which hotel to stay. In C, similar conditions may appear where we will have to find out the decision from number of alternatives. C provides a special control statement that allows us to handle such cases successfully rather than using a series of if statements.
The control statement that allows us to make a
decision from the number of choices is called a
switch.
• The Switch Statement:
The control statement that allows us to make a decision from the number of choices is called a switch.
The switch statement helps to make a decision from the number of choices. The switch statement tests the value of a given variable (or expression) against a list of case values and when a match is found, a block of statements associated with that case is executed.
A switch statement allows a variable to be tested for equality against a list of values. Each value is called a case, and the variable being switched on is checked for each switch case.
The integer expression following the keyword switch is any C expression that will yield an integer value. It could be an integer constant like 1, 2 or 3, or an expression that evaluates to an integer. The keyword case is followed by an integer or a character constant. Each constant in each case must be different from all the others.
The “do this” lines in the above form of switch represent any valid C statement.
What happens when we run a program containing a switch?
1.The integer expression following the keyword switch is evaluated.
2.The value it gives is then matched, one by one, against the constant values that follow the case statements.
3.When a match is found, the program executes the statements following that case, and all subsequent case and default statements as well.
4.If no match is found with any of the case
statements, only the statements following the
switch(expression) {
case constant 1 : statement_1;
break;
case constant 2:
statement_2;
break;
………..
………..
case constant n:
statement_n;
break;
default:
default_statement_x;
break;
• The expression is an integer expression or characters. constant 1, constant 2 …..etc.
are constants and are known as case labels. Each of these values should be unique within a switch statement.
statement_1, statement_2,………….
statement_n are statement lists and may
contain zero or more statements. The case
labels must end with a colon (:).
• When the switch is executed, the value of the expression is successfully compared against the values constant_1, constant_2…….constant_n. If a case is found whose value matches with the value of the expression, then the block of the statements that follow the case are executed.
• The break statement at the end of the each statement signals the end of a particular case and causes an exit from the switch statement.
• The default works same as else or if else
statement. It will be executed if the value of the
expression does not match with any of the case
The following rules apply to a switch statement :
• The expression used in a switch statement must have an integral or enumerated type, or be of a class type in which the class has a single.
conversion function to an integral or enumerated type.
• You can have any number of case statements within a switch. Each case is followed by the value to be compared to and a colon.
• The constant-expression for a case must be the
same data type as the variable in the switch, and it
• When the variable being switched on is equal to a case, the statements following that case will execute until a break statement is reached.
• When a break statement is reached, the switch terminates, and the flow of control jumps to the next line following the switch statement.
• Not every case needs to contain a break. If no break appears, the flow of control will fall through to subsequent cases until a break is reached.
• A switch statement can have an optional default case, which must appear at the end of the switch. The default case can be used for performing a task when none of the cases is true. No break is needed in the default case.
Flow Diagram
#include <stdio.h>
main () {
int score, grade;
printf (“Enter your score\n”);
scanf(“%d”, &score);
grade = score/10;
switch(grade) {
case 10:
case 9 :
printf("\n Grade is A" );
break;
case 8 :
printf("\n Grade is B" );
break;
case 7 :
printf("\n Grade is C" );
break;
case 6 :
printf("\n Grade is D" );
break;
default :
Disadvantages of switch :
The disadvantage of switch is that one cannot have a case in a switch which looks like:
case i <= 20 :
All that we can have after the case is an int constant or a char constant or an expression that evaluates to one of these constants. Even a float is not allowed.
Advantages of switch:
The advantage of switch over if is that it leads to a
more structured program and the level of indentation is manageable, more so if there are multiple
statements within each case of a switch.
Nested switch Statements
It is possible to have a switch as a part of the statement sequence of an outer switch. Even if the case constants of the inner and outer switch contain common values, no conflicts will arise.
The syntax for a nested switch statement is as follows:
switch(ch1) { case 'A':
printf("This A is part of outer switch" );
switch(ch2) { case 'A':
printf("This A is part of inner switch" );
break;
case 'B': /* case code */
}
break;
case 'B': /* case code */
Example
#include <stdio.h>
main () {
int a = 100;
int b = 200;
switch(a) { case 100:
printf("This is part of outer switch\n", a );
switch(b) { case 200:
printf("This is part of inner switch\n", a );
} }
printf("Exact value of a is : %d\n", a );
The ?: operator
The general form of the conditional operator is as follows:
conditional expression? true result : false result Let us understand this with the help of a few examples:
int x, y ;
scanf ( “%d”, &x ) ; y = ( x> 5 ? 3 : 4 ) ;
It has the following general form:
Exp1 ? Exp2 : Exp3;
This statement will store 3 in y if x is greater than 5, otherwise it will store 4 in y.
The equivalent if statement will be, if ( x > 5 )
y = 3 ;
else
y = 4 ;
It has the following general form:
Exp1 ? Exp2 : Exp3;
Where Exp1, Exp2, and Exp3 are expressions.
Notice the use and placement of the colon.
The value of a ? expression is determined like this:
1. Exp1 is evaluated. If it is true, then Exp2 is evaluated and becomes the value of the entire ? expression.
2. If Exp1 is false, then Exp3 is evaluated and its
value becomes the value of the expression.
The goto statement
The goto statement causes your program to jump to a different location, rather than execute the next statement in sequence. The format of the goto statement is;
goto statement label;
Consider the following program fragment
main() {
int a,b;
d:scanf("%d",&a);
if(a<5) goto d;
b=a+5;
Thanks to Everyone
See you in next
class.
